The primary goal of this research is to advance understanding of the development of cognitive flexibility. Children can show stunning failures to flexibly adapt their behavior to changing circumstances. For example, children often perseverate, repeating prior behaviors when they are no longer appropriate, even in the face of repeated and explicit information to change course. In contrast, mature humans show unique abilities to flexibly adapt their behavior to changing circumstances, even in the absence of explicit information about how or when to do so. Much of the work in the developmental literature has focused on one aspect of this process - children's increasing abilities to actively maintain goals that are provided for them. While this work has led to a rich understanding of the mechanisms supporting children's flexible behaviors, it has largely failed to address two critical transitions in development. First, prior to children's gradual improvements in proactive maintenance of goal representations, they show a qualitatively different, reactive, form of cognitive control, retrieving goals only as they are needed in the moment. Second, even after children excel at maintaining goals that are provided for them to guide exogenous (externally-cued) flexibility, they show continued limitations and developments in endogenous (internally-driven) flexibility, when they must select goals for themselves. The proposed research builds on a unified, biologically-based computational modeling framework to investigate developments in these diverse components of flexibility, through parallel studies with children and neural network models. This framework focuses on the role of active, abstract knowledge representations in driving transitions from reactive to proactive control, and from exogenous to endogenous control. This work will advance explicit mechanistic models of how children develop, maintain, and update representations in the service of flex- ible behavior, and will inform applications for improving flexibility - from infant search to children's rule use to increasing abilities to drive flexible behavior on one's own.